It is well established that laser irradiation can color the exposed surfaces of various materials by forming an oxide or nitride layer (referred to herein as a ‘thin film’ or ‘color layer’ or ‘color pattern’). See M. Wautelet, Appl. Phys. A 50, 131 (1990). Fundamentally, surface colorization of metals and alloys is a thermochemical growth process facilitated by the heat of absorbed laser light. Localized heating, provided by a focused laser beam, can lead to the growth of dielectric phases with the thickness of these thin films related to the dwell time of the laser and the absorption and diffusion of reactive gas (e.g., O, N, other). The color that forms is intimately related to the layer thickness and its refractive index. If illuminated by white light, the color can be the convolution of reflected wavelengths and those that interfere constructively; wavelengths that interfere destructively will be absent.
The majority of previous experimental and theoretical work on laser colorization involves continuous wave (CW) laser exposure. The extensive work by Wautelet and other groups demonstrates the ability to synthesize oxide layers on more than 20 simple metal surfaces. See M. Wautelet, Appl. Phys. A 50, 131 (1990); M. Wautelet and R. Andrew, Philos. Mag. B 55, 261 (1987); M. Wautelet and L. Baufay, Thin Solid Films 100, L9 (1983); M. Wautelet et al., App. Phys. A 47, 313 (1988); M. H. Wong et al., Materials Letters 61, 3391 (2007); and I. Ursu et al., J. Mod. Opt. 34, 1121 (1987). Chemical kinetic models have been developed to explain the rates of color layer growth during CW irradiation of elemental metals and the dynamical effects of a changing optical absorption coefficient as thickness increases. Studies have also correlated optical properties (often determined by ellipsometry) with thickness and structure. See A. Pérez del Pino et al., Surf. & Coatings Tech. 187, 106 (2004); and A. Pérez del Pino et al., Appl. Phys. A 78, 765 (2004).
Recently, several groups have investigated how pulsed laser light interacts with metal surfaces. See E. György et al., Appl. Phys. A 78, 765 (2004); M. H. Wong et al., Mater. Lett. 61, 3391 (2007); L. Lavisse et al., Laser in Eng. 13, 127 (2003); B. S. Yilbas et al., J. Mat. Proc. Tech. 136, 12 (2003); E. György et al., Surf. & Coatings Tech. 187, 245 (2004); E. György et al., Proc. SPIE 5581, 323 (2003); E. György et al., Appl. Phys. A 78, 765 (2004); and A. Pérez del Pino et al., Thin Solid Films 415, 201 (2002). Generally, this process involves sequential irradiation of a metal surface (or evolving color layer) with a train of light pulses. Models and experiments have focused on predicting the thickness of color layers by accounting for the repetitive rise in temperature, the cooling rates (after the arrival of a pulse) and the gas diffusion coefficient into an irradiated target material. See B. S. Yilbas et al., J. Mat. Proc. Tech. 136, 12 (2003).
Surface roughness is an additional feature that can develop during laser irradiation. A scanned-laser process can produce a periodic surface roughness due to the point-to-point variations in laser fluence incident on a surface. The variation in fluence across a surface can lead to surface roughening, because material removal (by ablation, evaporation or sublimation) occurs at sites where fluence is large compared with neighboring low fluence areas. For CW laser and pulsed laser processes, a rough final surface can arise from the spacing (hatch) between subsequent lines commanded by the operator. By choosing a hatch that is significantly less than the width of the laser beam, one generally establishes a uniform fluence across an area. By choosing a hatch that is significantly larger than the width of the laser beam, one establishes a non-uniform fluence. For pulsed laser irradiation, the arrival of individual pulses can further lead to roughening. Depending on the relative rates of pulse arrival and scan velocity, a variety of surface morphologies can develop.
Previous work has shown that ultra-fast lasers (having pulse durations of approximately <1 picosecond) can induce periodic surface structures. The periodicity can be on the order of or less than the laser wavelength. The open literature points to ablation as a pathway for roughening although a number of fundamental interactions have been proposed. Since the original report by Birnbaum, laser-induced periodic structures have been researched for their regular spacing and ubiquitous nature—forming in several monolithic solids including semiconductors and dielectrics. See M. Birnbaum, J. Appl. Phys. 36, 3688 (1965). The polarization of the incident laser light may give rise to periodic surface roughness in some processes.